The present invention relates to CMOS amplifiers useful for crystal oscillators.
In the past, a CMOS analog amplifier as illustrated in FIG. 1 was employed as an amplifier in crystal oscillators for a variety of electronic apparatuses for example solid state wristwatches to fulfill low power consumption requirements. The amplifier includes a basic inverter consisting of a P-channel MOS transistor P and an N-channel MOS transistor N, and a resistor R.sub.f of a high resistance value connected between an input terminal V.sub.in and an output terminal V.sub.out of the complementary MOS inverter for biasing purposes. Advantageously, the circuit exhibits high gain, low power dissipation performance characteristics while retaining simple circuit construction.
Nevertheless, one difficulty is experienced in reducing the output impedance in the vicinity of the operating center of the CMOS inverter. If the output impedance is forced into a low impedance state, there is a requirement that the mutual conductance of the P-channel and N-channel MOS transistors be increased while the threshold level voltage V.sub.th thereof be decreased. Those requirements result in an increase in dissipation of current. In addition, because the output impedance is constructed by active elements such as MOS transistors, the output impedance is greatly varied in accordance with input and output potential levels, power supply voltages, temperatures, etc.
FIG. 2 depicts interrelation among an input voltage V.sub.in, an output voltage V.sub.out, output impedance R.sub.out, current I.sub.c flowing through P- and N- MOS transistors when supplied with a voltage (V.sub.DD -V.sub.SS), which occur within the CMOS analog amplifier of FIG. 1.
The graph (I) in FIG. 2 shows a V.sub.in -V.sub.out characteristic of a conventional CMOS transistor, indicating that V.sub.in is biased at the substantially same as V.sub.out via the bias resistor R.sub.f and thus the CMOS operating center V.sub.coc is given. The graph (II) shows the dependency of the output impedance R.sub.out upon variations in the output voltage V.sub.out, indicating that R.sub.out tends to take a maximum at the operating center V.sub.coc and reduce when the power supply potential approximates either V.sub.DD or V.sub.SS. Analysis of the graph (II) also reveals the fact that a reduction in the power supply voltage (V.sub.DD -V.sub.SS) and a temperature rise permit the output impedance R.sub.out to be increased as suggested by the curves (1) and (2).
The graph (III) shows variations in the current I.sub.c flowing through the CMOS when supplied with (V.sub.DD -V.sub.SS), as a function of the input potential V.sub.in. While the current I.sub.c has a peak at the operating center, the same is correspondingly reduced in accordance with a degree of deviation from the operating center V.sub.coc.
FIG. 3 shows an example wherein the above discussed type of the amplifier is applied to timekeeping crystal oscillators having a frequency of about 32.768 KHz. A quartz resonator is denoted as X and phase shifting or frequency adjustment capacitors are denoted as C.sub.1 and C.sub.2. When the crystal impedance value (hereinafter referred to as the CI of the quartz resonator X is relatively small (typically, in the order of several tens K.OMEGA.), the crystal oscillator of FIG. 3 is satisfactory for practical use of timekeeping devices. The critical conditions for practical use of timekeeping devices are that changes in oscillation frequency due to power supply voltage change .DELTA.f/f.V be below 1.0 sec/day.V, dissipation of current under a voltage supply of 1.6 V be below 5 .mu.A, the lowest oscillation initiating voltage be below 1.4 V and the oscillation initiating period be within 8 seconds.
However, if CI of the quartz resonator X is increased beyond 100 K.OMEGA., the circuit of FIG. 1 can fulfill these requirements no longer. In the event that CI is several hundred K.OMEGA., oscillation will be very difficult. It is believed that this is caused by the above disadvantages mentioned with respect to the CMOS analog amplifier of FIG. 1.
In a characteristic diagram of FIG. 2, the difficulty in reducing the output impedance Romax in the vicinity of the operating center V.sub.coc causes the period of time required for initiating oscillation to be considerably longer because it is difficult to excite the quartz resonator in order to initiate oscillation. In addition, the difficulty in reducing the average of the output impedance R.sub.out implies an increase in the oscillation initiating voltage is required.
The greater the variations in the output impedance R.sub.out of the amplifier as a function of power voltage variations, the greater the regulation of the oscillation frequency. Although silver batteries recommended for use in solid state wristwatches can provide a comparatively stable discharge voltage when loaded with a light-load, their temperature dependencies vary from battery to battery to thereby adversely effect the performances of the oscillator.
When the CI of the quartz resonator is increased in this way, the attenuation factor of a feed-back system including a phase shifter is correspondingly increased. As a consequence, the difficulty in initiating oscillation is still not avoided since the maximum and average of the output impedance R.sub.out of the amplifier remain unchanged at their high values.
Accordingly, it is a primary object of the present invention to provide an improved amplifier suited for effectively exciting quartz resonators of relatively high CI, which has the advantage of comparatively high gain, comparatively low output impedance, minimized variations in output impedance, and minimized power dissipation.